Energy seal for high frequency energy apparatus

ABSTRACT

A high frequency oven having a door sealed to the oven by a seal which prevents the escape of high frequency energy between the door and the oven by acting as a choke to energy attempting to pass across the seal in which excitation of energy modes in a band of frequencies including the desired operating frequency range of the oven is controlled peripherally along the seal. An energy absorbing gasket surrounds the seal to absorb any energy passing outwardly from the energy seal. 
     The questions raised in reexamination request No. 90/000,945, filed Jan. 29, 1986, have been considered and the results thereof are reflected in this reissue patent which constitutes the reexamination certificate required by 35 U.S.C. 307 as provided in 37 CFR 1.570(e).

.Iadd.This is a file wrapper continuation of application Ser. No.05/582,324 filed May 30, 1975 which is a reissue of U.S. Pat. No.3,767,884. .Iaddend.

BACKGROUND OF THE INVENTION

The invention relates to electromagnetic energy supporting structuresand more particularly to microwave heating apparatus having energy sealsto prevent the leakage of such energy from the apparatus. In themicrowave oven art the escape of high frequency energy from an ovencavity is desirably controlled in order to comply with standardsestablished by State and Federal regulatory agencies and bodies, such asthe Department of Health, Education and Welfare, Federal CommunicationCommission, and The United States of America Standards Institute.Conventionally, such apparatus operates at assigned frequencies ofeither 915 or 2,450 MHz and the term "microwave" as used in thisdescription of the invention is intended to refer to that portion of theelectromagnetic energy spectrum having wavelengths in the order of 1meter to one millimeter and frequencies in excess of 300 MHz.

In microwave ovens the energy fed to the oven interior preferablyresonates in a plurality of modes achieved by suitable adjustment of theoven dimensions. Such resonant modes are loaded by absorption of heat bythe article being heated, and such absorption varies with the absorbingcharacteristics of the article as well as its size and shape. To assureuniformity of heating it has generally been desirable to cyclically varythe mode pattern with respect to the article by, for example, amechanical mode stirrer, by movement of the article within the oven, byvarying the frequency of the energy fed to the enclosure, and/orcombinations of all the foregoing. The multiplicity of modes which varywith the loading within the oven and with the cyclical variation of themodes therein can result in excitation of modes within a door seal whichhave propagating components along the seal and can produce undesirablylarge amounts of energy leakage through the door seal.

In the prior art numerous energy seals have evolved including thoseproviding metal-to-metal contacting surfaces or interdigital structures,such as, for example, the embodiments referred to in U.S. Pat. Nos.2,956,143 issued to L.H. Shall on Oct. 11, 1960, 2,958,754 issued toD.E. Hahn on Nov. 1, 1960, as well as 3,448,232 issued to J.H. Kluck onJune 3, 1969. Among the disadvantages with such structures is themechanical variations which over a period of time develop gaps withsubstantial energy leakage. It is also inherent in such structures and,in particular, the last named structure that propagation of energy inall modes and in all directions along the energy seal must be suppressedby substantially complete metal-to-metal contact in order for the sealto be effective. Additionally, with the establishment of gaps overperiods of time between the contacting metal surfaces high frequencyenergy arcing may occur during operation and/or upon opening of thedoor.

Other prior art energy seals include electrical choke arrangementstogether with dielectric bodies to define paths of least resistance forenergy leaking along the peripheral gap defined between the door andaccess opening walls of an oven. Several examples of choke energy sealsare illustrated in U.S. Pat. Nos. 3,182,169 issued to Richard Ironfieldon May 4, 1965, and 3,584,177 issued to Arnold M. Bucksbaum on June 8,1971. The choke type energy seals are intended to handle single energypropagating modes and have, for example, a dimension of one-quarter of awavelength of the operating frequency of a TEM-mode along a first pathwith a total excursion of one-half a wavelength from a short circuitdefined by a terminating wall which is reflected back to the point oforigin of the escaping energy through the peripheral gap.

In previous embodiments such choke arrangements in the course of timemay change their electrical characteristics due to mechanical wear andbuildup of food particles which alters the door dimensions and hindersthe effectiveness of the choke in preventing leakage. As a result,heating apparatus which initially meets the low radiation standardlevels after installation may significantly drift from such levels. Ithas further been noted that with the excitation of plural modes withinthe oven cavity the region defined by the peripheral gap around theaccess opening becomes an efficient propagating structure for,particularly, modes propagating in a y-direction peripherally around thegap, as well as such energy modes intended to be directed across theenergy seal, hereinafter referred to as the x-direction, to encounterthe choke arrangement. A wide range of higher-order modes can beinitiated which propagate in the x-direction having wavelengths greatlydifferent than the operating frequency wavelength because these modesare associated with cutoff resonance of y-propagating modes atfrequencies close to the operating frequency.

In attempting to understand the electrical characteristics of theundesired modes propagating peripherally along the energy sealassociated with the x-directed modes a study of the longitudinal andtransverse current flows has indicated that energy seals, particularlythose with choke arrangements, exhibit large quasi-periodic variationsof leakage superimposed upon or even counteracting the expected TEM-modesuppression characteristics of the choke. Energy absorbing bodies, suchas gaskets, are, therefore, coupled just beyond or outside the modesupporting energy seals in order to further dissipate energy leakage andassure maintenance of the standards. If the dielectric bodies within thechoke or the energy absorbing bodies do not have stable dimensions andlocations during the operation of the oven apparatus or if the doorlocation and centering varies then excessive energy leakage and gasketheating will occur in an uncontrollable manner. A need arises,therefore, for an improved high frequency energy apparatus having anenergy seal which will provide for controlled energy mode propagation inone direction by inhibiting the initiation of modes which can propagatein a different direction.

SUMMARY OF THE INVENTION

The present invention provides for an electromagnetic energy modesupporting structure having means for controlling undesired propagationmodes. More specifically, means are provided defining an energypropagation path with energy being propagated in one direction butprevented from propagating in a different direction. As used throughoutthe specification and claims, the term "mode" means a state ofelectromagnetic wave energy characterized by a particular distributionin space and time. The term "propagation" is intended to meanenergization with an electromagnetic wave having a real phase velocityand is further intended to include standing and/or non-standing waves.The term "real" relating to the phase velocity is intended to refer to awave which is not cut off from propagating and is capable of propagatingwithout substantially decaying.

The invention further provides that such a mode supporting structure canbe used to prevent the escape of energy around the peripheral gap of anoven door. Such an improved energy seal is preferably positionedadjacent to at least one of the peripheral walls adjacent to theopening. The mode supporting structure is provided with circuitparameter variations along at least a portion of the structure such thatthe parameters have maximum and minimum points spaced apart less thanone effective electrical wavelength at the operating frequency. The term"circuit parameter," as used herein in the specification and claims,means any of those circuit elements or waveguide constants which aredependent on geometry and/or material which in a generalizedtransmission line mode affects the characteristics of propagation ofstanding electromagnetic wave in a structure.

One wall of such a mode supporting structure is movable with respect toan opposing wall when the door is opened. To inhibit the excitation ofmodes propagating in the longitudinal or peripheral direction around theaccess opening, periodic and/or aperiodic spaced structures such asslots are provided which yield stop bands of frequencies which preventresonances close to an operating frequency. The absence of suchresonances results in the absence of modes propagating transverse oracross the mode supporting structure with cutoff resonant frequenciesclose to the operating frequency. This then further results in therestriction of guided wavelength values of all modes propagating in thetransverse direction to values close to a single value. Such circuitparameter variations may be provided in at least one of the walls of themode supporting structure formed by the door or, alternatively in atleast a portion of the access opening walls.

The invention further provides for the use of an elongated energy sealin heating apparatus to prevent substantial leakage of energy from theaccess aperture used to introduce material to be heated into the ovenenclosure. By inhibiting the propagation of energy along the energy sealand thereby restricting propagation to a direction across suchstructure, an electrical choke may be constructed which reflectssubstantially all of the energy entering through the peripheral gap andsubstantially reduces or eliminates the need for energy absorbing bodiesat the primary operating frequency of the oven surrounding the door. Thereduction of the absorption requirements of the energy absorbing bodiespermits such bodies to be used effectively to absorb any harmonics ofthe operating frequency which may be inadvertently generated by thesource of energy.

The invention may be utilized in numerous other electromagnetic energyapparatus and systems where control of propagation of energy modes isrequired for efficient operation.

BRIEF DESCRIPTION OF THE DRAWINGS

Details of illustrative embodiments of the invention will be readilyunderstood after consideration of the following description andreference to the accompanying drawings, wherein:

FIG. 1 is a perspective view, partially broken away, of a portion of anoven and control system embodying the invention;

FIG. 2 is a longitudinal cross-sectional view of the oven portion of theembodiment of the invention illustrated in FIG. 1 taken along the line2--2 in FIG. 3;

FIG. 3 is a partially broken away cross-sectional view of saidembodiment taken on the line 3--3 in FIG. 2;

FIG. 4 is an enlargement of a detail of the cross-sectional view of FIG.2 illustrating a portion of the choke-type door seal arrangement in saidembodiment taken along the line 4--4 in FIG. 2;

FIG. 5 is a cross-sectional view of the detail of FIG. 4 taken along theline 5--5 in FIG. 4;

FIG. 6 is a schematic representation of the equivalent circuit of theembodiment of the invention shown in FIGS. 1-5;

FIG. 7 is a diagram illustrating the ω-β characteristics for a modesupporting structure having means for variation of the circuitparameters;

FIG. 8 is a plot of the curves of energy leakage versus frequencymeasurements of modified and unmodified heating apparatus;

FIG. 9 is a perspective view of an illustrative apparatus having aside-hinged door seal arrangement;

FIG. 10 is a cross-sectional view of a sliding door seal arrangement;

FIG. 11 is a partial cross-sectional view of an alternative embodimentof the invention for choke-type door seal arrangements;

FIG. 12 is a partial cross-sectional view of still another alternativechoke-type door seal arrangement embodying the invention;

FIG. 13 is a partial cross-sectional view of an alternative chokearrangement;

FIG. 14 is a partial cross-sectional view of an alternative modesupporting structure embodying the invention;

FIG. 15 is a cross-sectional view of another structure embodying obliquecircuit parameter variation means;

FIG. 16 is a partial cross-sectional view of another embodiment of theinvention providing corrugated circuit parameter variation means;

FIG. 17 is a partial cross-sectional view of an aperiodically structureembodying the invention;

FIG. 18 is a partial cross-sectional view of another embodiment of theinvention;

FIG. 19 is a partial cross-sectional view of an energy seal arrangementfor use in alternative choke seal systems;

FIG. 20 is a vertical cross-sectional view taken on the line 20--20 inFIG. 19 looking in the direction of the arrows;

FIG. 21 is a partial cross-sectional view of an alternative energy sealarrangement; and

FIG. 22 is a partial elevational view of another embodiment of the typeshown in FIG. 21.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1-5 inclusive, the heating apparatus 10 embodyingthe features of the invention is of the drop-down door type comprising ahollow cavity enclosure 12 defined by conductive walls 14. A case member16 surrounds the enclosure as well as the accompanying microwave energygenerator, electrical circuits and controls. A control panel 18 supportsa 5-minute timer switch 20 as well as a 30-minute timer switch 22. Astart control button 24, stop control button 26 and light control button28 are also mounted on panel 18. An upper wall member 14 is providedwith perforations 17 to facilitate the removal of any heat within thecavity enclosure.

A source of electromagnetic energy, such as a magnetron type oscillator30, is coupled to the high voltage supply circuits and electricalcontrols indicated generally by box 31. The energy is fed by means ofantenna 32 mounted within a dielectric dome member 34 into a launchingwaveguide section 36 adapted to transmit energy to the enclosure at thefrequency of desirably 2,450 MHz in principally the TEM mode. Thewaveguide section is short-circuited at one end by wall member 38 withthe opposing end 40 being tapered and open. The energy is distributed ina cyclically varying manner to produce a plurality of modes within theenclosure by, for example, a mode stirrer 42 having a plurality ofpaddle members 44 actuated by a motor 46 connected to shaft 47. Suchmode stirrers provide for the excitation of numerous modes at random aswell as planned orientations within the enclosure 12. The energy modedistributions have varied patterns and the oven cavity is dimensioned tobe substantially greater than an effective electrical wavelength of thepredetermined frequency, for example, 2,450 MHz. The items to be heatedare supported within the enclosure 12 by any suitable means such as, forexample, a tray of a dielectric material.

Access to the interior of the conductive enclosure 12 is provided bymeans of opening 50 closed by means of door assembly 52. In thedrop-down model a bottom hinge 54 is provided and the movement of thedoor assembly is controlled by a pair of spring tension counterbalancedarms 56. Safety interlock switches are contacted by arms 56 and arehoused adjacent to walls 14.

Door assembly 52 comprises a panel member 62 and ring member 64 securedtogether by any conventional means to form a unitary assembly to bedescribed. Perforations 66 in metal panel member 62 allow visualobservation of the oven interior during cooking while preventing theescape of any electromagnetic energy radiated within the enclosure. Anouter window member 61 is supported within ring 64. An inner windowassembly 63 with a transparent region also renders the perforationsinaccessible to damage and simplifies cleaning of the oven interior. Astud 65 secured to member 63 is press-fitted into apertures in panelmember 62. Window member 61 is retained in position by adhesive means aswell as door trim members omitted from this view for the sake ofclarity. A latch 68 is pivotally mounted on the door assembly 52 andengages a mating slot 70 in the peripheral front wall 72 surrounding theaccess opening. A mechanically actuated latch locking arrangement 74 isslidably disposed within the control panel 18 and is coupled to aninterlock switch to prevent opening of the door after the apparatus isoperative.

A door-type electrical choke arrangement 76 extends around the peripheryof the door to form an elongated seal, a portion of which is mounted onthe door assembly with the wall members having a slightly taperedconfiguration to mate with the tapered walls 15 of the conductive walls14 to assure a snug fit after closing. Referring now to FIGS. 4 and 5 anenlarged view of the choke arrangement is shown. Panel member 62 definesa peripheral common wall section 78 which defines with the opposingtapered conductive wall 15 an elongated electromagnetic energy path 80extending peripherally around the opening 50. The point of origin of theenergy is indicated at the gap 82. Ring member 64 has a substantial stepconfiguration to provide a conductive wall surface 84 forming a part ofthe choke arrangement as well as a front lateral member 86 overlappingthe casing 16 opposite to the peripheral walls 72. The door assembly isfabricated by welding or riveting or any of the well-known metalfabrication techniques for securing the abutting surfaces. Ring member64 defines with common wall section 78 a second electromagnetic energypath 88 for any energy escaping through the peripheral gap 82. Theparallel paths 80 and 88 may be filled with a dielectric material, suchas a body 90 of polystyrene or polypropylene. The entrance and exit tothe choke cavity path 88 is provided by the gap 92 defined between theend of the wall section 78 and the opposing wall 84. The foregoingarrangement is an efficient high frequency energy seal which offers apath of least resistance for any escaping energy around the peripheralgap 82.

In addition to the energy seal, elongated energy absorbing bodies 94 and96 may be disposed as shown between the wall surface 86 of the door ringmember 64 and the peripheral wall surfaces 72. Bodies 94 and 96 may beof any desired material for absorbing electromagnetic energy passingthrough the choke arrangement 76 including rubber or plastic materialsloaded with carbon derivatives or ferrite materials and the like and maybe secured to the metallic walls by suitable adhesive materials. Such anenergy absorbing seal also absorbs any frequency harmonics of theprimary frequencies (2,400-2,500 MHz) which may be generated by themagnetron.

In accordance with the principles of energy transmission such electricalchoke arrangements are provided with wall portions defining paths 80 and88 selected primarily to provide a high series reactance at the chokeopening and to reflect a short circuit from a terminating wall surface98 to the energy directed through gap 82. The choke dimensions aretypically selected to provide a short circuit at the point of origin ofthe escaping energy or approximately one-half a wavelength of theoperating frequency. It has been observed, however, in previousembodiments that energy may be transmitted along the seal in they-direction as indicated by the arrow 48 peripherally in the directionof the elongated path around the access opening. Such y-directed energymay have modes and/or wavelengths at the operating frequency differingfrom the desired wavelength and mode of the energy propagating acrossthe seal at said frequency in the x-direction as indicated by arrow 60.These modes and, particularly, certain higher-order modes may havecutoff resonant frequencies close to the operating frequency.

In accordance with the teachings of the invention a plurality of spacedcircuit parameter variations are provided by such means as capacitiveslots 106 in at least one of the choke wall sections 78 to definetherebetween substantially finger-like conductive members 78a.Substantial variations in the circuit parameters of the mode supportingstructure results with alternate maximum and minimum points of suchparameters. The distance between the points of maximum and adjacentpoints of minimum variation is less than one effective electricalwavelength of the frequency of excitation within the oven enclosure. Theillustrated slots have a major dimension extending in the x-directionand, therefore, at an angle to the y-direction or peripheral directionof the mode supporting choke seal structure. The underlying dielectricmaterial 90 within the choke structure path 88 is thereby exposed toenergy propagating across the elongated peripheral gap in thex-direction while the slots 106 act as open circuits to establish stopbands which inhibit initiation of energy mode propagation in they-direction.

The circuit parameter variations may be provided in a periodic oraperiodic manner or combination of both and the distances between theslots will provide any series of high reactances as long as thedimensions of the period are less than one effective wavelength of theenergy at the given frequency within the enclosure 12 preferably,substantially less than one-half of the wavelength of the operatingfrequency. Referring particularly to FIG. 5 it will be noted that theslots 106 provide alternate finger-like conductive member 78a. The widthdimension "a" of the slots should be great enough to present a low valueof capacitance to energy propagating across the elongated modesupporting structure in the x-direction. If the slot width is excessivedirect coupling between the energy in the oven enclosure and the chokepath 88 will occur. Dimension "b" is desirably less than one electricalwavelength including the effects of the dielectric loading present. Thelength of the slots 106, dimension "c", or the major dimension extendingnormal to the direction of modes to be inhibited in exemplaryembodiments was close to one-quarter of a wavelength including theeffects of junction susceptance. In the illustration, therefore, theslot dimension "c" is substantially the full length of the choke wallsection 78. Alternatively, the slot may terminate a short distance fromthe edge of panel member 62. The slots 106 have been illustrated asbeing void of any medium other than air, however, in certainapplications it may be desirable to fill the slots with dielectricmedium similar to that employed for the body member 90 disposed withinpaths 80 and 88.

While the reasons for the successful operation of the invention are notthoroughly comprehended certain general principles apply to structureshaving the elongated geometry of a typical heating apparatus doorassembly or any large waveguide structures capable of supportingnumerous propagating modes by referring first to the schematicrepresentation of the equivalent circuit depicted in FIG. 6. The seriesof periodic or aperiodic variations in the form of capacitancereactances are spaced a distance p with each slot having a capacitivevalue C to load a mode supporting structure having a characteristicimpedance Zo. Although the peripheral gap under consideration isreentrant, the structure may be considered for the sake of simplicity asbeing infinite in length in order to analytically deduce certaintheoretical principles which will assist in an understanding of theinvention.

Referring now to FIG. 7 the frequency characteristics of an infinitecapacitively loaded structure are plotted in terms of frequency ω andthe propagation constant β. In addition to the TEM-like modes within thepath 80 in the x-direction a number of possible modes in which theguided wavelength λg/λ is considerably different than unity can bestudied utilizing the principles of quasi or modified periodicstructures. The phase change across the capacitor C is such that phasevelocity varies to establish a series of stop and pass bands. Certainmodes, therefore, can be determined to have an ω/β relationship forpropagating in the y-direction indicated by line 116 and a slopecharacteristic dω/dβ shown by line 118 for curves 120, 122 and 124.Considering Maxwell's theory together with Floquet's theorem whichstates that for a given mode of oscillation in frequency the propagationwave function is multiplied by a constant complex factor on moving alongthe structure by one section or period, the cutoff frequencies forenergy propagating along the y-direction may be determined.

The analysis provides the realization that pass band 126 exists tosupport desired energy transfer. A stop band also exists as indicated byline 128 in a certain frequency range preventing energy transfer. Inaddition to the higher-order modes certain lower frequency modes mayalso be supported within a mode supporting structure as indicated bycurve 130. If a mode supporting structure is provided along they-direction with circuit parameter variations which will substantiallyeliminate cutoff resonances composed of y-directed energy, then anymodes with cutoff frequencies for propagation in the x-direction closeto the operating frequency will be inhibited. Energy modes can thenpropagate in the x-direction with λg˜λ. Whereas previously somehigher-order modes would have propagated in the x-direction close to theoperating frequency, now these modes can initiate only reactive orevanescent penetration along the x-direction with insignificant couplingof the oven enclosure fields to external space through the gap region.

The invention teaches the establishment of such circuit parametervarying means to inhibit the initiation of modes propagating in theperipheral or y-direction by providing a stop band free of cutoffresonances close to the operating frequency f_(o). The range of between0.5 and 1.5 of the operating frequency, indicated between the dottedlines 132 and 134 in FIG. 7, has an approximate three-fold range toyield very effective inhibition of modes in the y-direction.

Classical periodic structure theory, for example enumerated in the text"Microwave Engineering" by Harvey, Academic Press, New York and London,at p. 436, yields the following equation for the propagation constant βof energy in the peripheral or y-direction:

    cos βp=cos β.sub.o p+1/2ωCZ.sub.o sin β.sub.o p (1)

where β_(o) =2π/λis the propagation constant of the energy in the modesupporting structure having an electrical wavelength including theeffect of dielectric loading. Zo is the characteristic impedance and "p"is the spacing between consecutive capacitances C. The upper edge of thestop band can be made close to β _(o) p ˜π if ωCZo is substantiallysmall, for example, ωCZo<<1. If the stop band is made to be at leasttwice 2f_(o) then there can be no cutoff resonance in the y-directionfor x-propagating modes at all frequencies below 2f_(o). The essentialcondition for the inhibition of higher-order mode propagation is,therefore, fulfilled by selecting a slot width to yield a lowcapacitance C and a small period p. In actual embodiments operating at2,450 MHz a "p" dimension in the range of 0.650-0.750 inches and an "a"slot dimension in the range 0.070-0.125 inches provided desiredpropagation characteristics.

Before proceeding to a description of numerous other embodiments ofperiodically or aperiodically circuit parameter variations theperformance measurements of oven leakage versus frequency illustrated inFIG. 8 will now be described. The illustrative curves represent a largenumber of readings taken at numerous locations around the periphery ofthe energy seal with a large number of curves superimposed and themaximum envelope taken as the curve of maximum leakage versus frequencyfor each particular combination. Curve A represents a microwave ovendoor seal without any electrical choke arrangements or modifications.Curve B represents an oven door assembly with just a choke. Curve Crepresents a door assembly modified in accordance with the invention bycapacitive slots having widths in the range of 0.070-0.125 inches and aspacing "p" in the range 0.650-0.750 inches together with an electricalchoke arrangement. All three curves reflect results without the use ofany secondary lossy or damping gaskets, as well as plastic materialscovering the perforations in the door. The frequency f_(o) is theassigned operating frequency of microwave oven apparatus.

Curve A without any protection indicates a high level of leakage whichdecreases with frequency according to theoretically confirmed variationof input reflection. A choke arrangement indicated by curve B reducesthe leakage by about 10-20dB with no distinct choke resonance indicatedin the vicinity of f_(o). It is believed that this lack of distinctchoke resonance results from the uninhibited higher-order modes alongthe elongated peripheral path which is in large measure unaffected bythe choke action since λg/λ is far from unity. The expected choke peaksuppression at the operating frequency of x-directed modes is,therefore, masked. With the addition of the appropriate circuitparameter variations to establish a stop band for energy propagating inthe y-direction, distinct evidence of choke action is revealed by curveC at the resonance point f_(o), indicated by the numeral 140. It mayalso be noted that the minimum leakage at the f_(o) point isapproximately 20dB below the leakage of an unmodified choke doorindicated by curve B. The frequency dependence has also been observed tobe noncritical with only a 10dB degradation in choke performance for a±200 MHz frequency deviation from choke resonance 140. As a result,electrical deviations during the life of an apparatus due to mechanicalvariations in embodiments of the present invention will certainlymaintain satisfactory safety standards over extended periods of time.

It has also been noted in actual tests that the addition of the lossyenergy absorbing bodies typically lowers the maximum leakage by another10-15dB in unmodified doors whereas the addition of such material lowersthe leakage by only 5-10dB with the door modified in accordance with thepresent invention. This data indicates that the door seal arrangement ofthe invention relies less on the additional lossy absorbing materials byreason of the inhibiting of higher-order mode energy propagation. Theinvention restores the normal function of the electrical chokearrangement in primarily controlling the TEM modes propagating in thex-direction across the peripheral gap. The data, therefore, indicates animprovement factor of 40-100 in reducing the leakage levels well belowlevels achieved with existing oven door arrangements.

A modification of the invention is disclosed in FIG. 11 wherein spacedslots 108 are now provided in wall 15. Choke arrangement 76 in thisembodiment provides for the wall section 78 of panel member 62 to remainunmodified. The capacitive slots, therefore, are provided in wall 15 toresult in the inhibition of the desired modes. The energy propagating inthe y-direction coupled through slots 108 may be attenuated by suitablemeans within chamber 110 defined with case 16 by an end wall 111.

In FIG. 12 an alternative door seal arrangement is disclosed with choke142 provided as a part of the enclosure wall 14 instead of the doorassembly. The dielectric body 90 and transmission paths 80 and 88 aredefined similar to the preceding embodiments. Panel member 144 and ringmember 146 are fabricated as a single unit to define the overall doorassembly 148 which contacts the wall choke arrangement. The choke wallportion 152 having the spaced capacitive slots 154 is secured bysuitable means to oven wall member 14. The resultant door-chokearrangement is again provided around the access opening.

In the foregoing descriptions the inner and outer door window members 61and 63 have been shown. It may also be noted that while a chokearrangement has been described, in certain embodiments of modesupporting structures the gaps between the door assembly and ovenenclosure walls may be loaded with magnetized ferrite bodies to perturbthe escaping energy. In such embodiments capacitive structures can bearranged in the oven enclosure walls in essentially the same mannershown in FIG. 11.

Other circuit parameter variation structures will now be described. InFIG. 14 slots 156 and 158 having varying lengths c₁ and c₂ providesubstantially U-shaped members 160. In this embodiment one set ofcapacitive reactances can inhibit one frequency mode while the secondset inhibits another frequency. In FIG. 15 slots 162 extend obliquely todefine therebetween conductive members 164. In all the foregoingexamples, the modification of the wall surfaces can be practiced foreither the wall defining the enclosure or the door. In some embodimentsboth walls provide the requisite circuit variations.

In FIG. 16 a variation of the invention is disclosed with the corrugatedwall 166 comprising a plurality of conductive rib sections 168 toprovide alternating high and low impedance regions around the accessopening. The ribs extend parallel to the x-direction or transverseacross the elongated peripheral path. Dielectric material 170 isdisposed in a similar manner to the previous embodiments. Spaced highimpedance regions are defined by channels 172 between the rib sections168 in the low impedance region. In this embodiment the x-directionextends in the direction indicated by the circle. The y-direction isagain indicated perpendicular to this direction. A z-direction is alsoindicated showing the direction of energy into the choke filled withdielectric 170. In this embodiment further improvement in inhibitingmodes may be realized by the provision of slots in the back walls ofeach channel as indicated by dashed lines 171.

FIG. 17 illustrates an aperiodically varied structure having somewhatrandom spacings with all the periods being much less than one effectiveelectrical wavelength long, for example all less than λ/4 of thepredetermined operating frequency. Hence, the distance between slots 174and 176 provides a first spacing p₁. The distance between capacitiveslots 176 and 178 provides a second spacing p₂. Similarly, the distancebetween slots 178 and 180 provides a third variable spacing. Theconductive wall members disposed between the slots form varying spacedconductive members 182, 184 and 186.

FIG. 18 illustrates a choke arrangement 76 similar to that shown inFIGS. 4 and 5 modified by insertion of a dielectric or other desiredmaterial 107 in slots 106 to aid in achieving the desired circuitvariations.

An alternative heating apparatus of the side-hinged type is shown inFIG. 9. Access opening 190 is enclosed by means of door assembly 192having a latch member 194. Control panel 196 includes a mechanicallatching-interlock arrangement 198 which engages a slot 200 in the sidewall of the door assembly. The latch member 194 engages slot 202 inperipheral wall 204 surrounding the access opening. Safety interlockswitches are supported by plate 206 positioned between enclosure memberwall 208 and the inner oven wall 210. Door assembly 192 is provided witha series of circuit variations to provide the desired inhibitingcharacteristics by slots 212 in the side wall of the door choke assemblyto provide therebetween spaced conductive members 214. In thisembodiment a cover 216, with a window of a plastic material, encases theinner wall portions of the door arrangement. The distribution of thecircuit variations again follows the principle of the teachings of theinvention as explained in detail with regard to the illustrations of thedrop-down apparatus. A secondary absorbing seal is provided by any meanssuch as, for example, a graphite loaded gasket 193 having a thickersection 195 adjacent to the choke dielectric material 197. The doorassembly is completed by perforations 199 and in this embodiment thesurface of wall 204 may be left uncovered by lossy gasket material toexpose bare metal.

In FIG. 10 a slidably disposed door assembly 240 embodying the inventionis illustrated comprising an outer skin member 242 of a metal, such asaluminum, wrapped around a frame 244 which may consist of sections of ametal channel seam-welded to form a picture frame-like structure toenclose access opening 246. Handle 248 is attached to the door assemblyfor manual operation. Track mechanisms 250 and 252 include nylon-bearingsurfaces 254 and 256 inserted on the outer edge of a flange of the ovenapparatus wall 258. Guide rails 262 and 264 are wrapped around thisstructure and are coupled between the outer skin member 242 and frame244 by means of screws 266. Dielectric spacers 268 are inserted into theframe 244 and provide a gap 270. An inner skin member 260 of a suitablemetal or other material encloses the inside of door frame 244 toreinforce the door.

Frame 244 defines a choke structure 272 having interior walls 274, 276,278 and 280. Gap 270 as well as choke structure 272 may be filled with adielectric material if desired. Wall 274 presents a terminating shortcircuit which reflects back to the point of origin of energy escapingthrough gap 270. Choke wall 280 and the opposing wall surface 282 of theoven wall 258 form a boundary of gap 270 which is effectively a modesupporting line. In this embodiment the x-direction in which theTEM-like modes are propagated is indicated by the arrow 284. The circle286 indicates the y-direction of the modes to be suppressed and extendsperpendicular to the x-direction. In accordance with the invention,choke wall 280 is appropriately modified by the provision of capacitiveslots 288 having a major dimension extending normal to the y-directionof propagation. The foregoing embodiment may further be arranged withthe choke structure 272 disposed within oven wall 258.

FIG. 13 illustrates a door arrangement 400 closing an opening in ovenenclosure 402 defined by conductive walls 404. A frame 406 havingperforations 408 and plastic window cover 410 is provided with anabsorption cavity having walls 412 and 414 filled with a dielectric body416. Energy escaping from enclosure 402 traverses the gap between wall404 and wall 418 of the frame to first enter the absorption cavity. Theenergy path then extends right angularly along the front walls of theenclosure. An energy absorbing material 419 is disposed with a resilientwall member 420 adjacent the outer edge of the door. A trim moulding 422and 424 completes the door assembly.

In accordance with the invention either wall 418 or opposing wall 404 ismodified by slots 426. In this illustration these slots have been shownin wall 218. The dimensions are selected to inhibit propagation ofenergy in the peripheral elongated path designated by the circle 428 andy-direction while the x- or desired energy path is designated by arrow430. All the variations previously described can also be embodied inthis structure.

FIGS. 19 and 20 illustrate a waveguide transmission line capable ofsupporting numerous modes. Coupling flanges having a choke wall 218provide two trough-like structures 220 and 222 within which transverseor oblique modes may be propagated while the direction of a describedtransmission is indicated by the arrow 224. Coupling flange members 226and 228 couple waveguide sections 230 and 232. In this embodiment thecircuit parameter variations are defined by slots 234 in choke wall 218.Again inhibition of peripherally directed propagating modes will resultin a more efficient single mode transmission along the main line.

FIGS. 21 and 22 illustrate another embodiment of a door seal arrangementproviding an elongated propagation line comprising bars having widths ofapproximately one-quarter of a wavelength extending longitudinally or,what has hereinbefore been referred to as the y-direction, peripherallyadjacent to the access opening walls with dielectric materials spacedbetween alternate bars. Each of the propagation lines are provided witha terminal end to reflect a low impedance at the point of energy origin.Explicit details regarding this embodiment may be had by referring toU.S. Pat. No. 3,511,959 issued May 12, 1970, to J. R. White.

In FIG. 21 the partial cross-sectional view discloses a conductive wallmember 300 forming an oven enclosure 302. A mode supporting structure304 includes first and second bar members 306 and 308 secured to surface310 of the front conductive wall 300 around the access opening 312. Theconductive bars are spaced apart to provide a space 314 of highimpedance characteristics. Movable conductive panel 316 of door 318provides a boundary wall member to form with wall surface 310, thestructure 304 for transmission of energy having its origin adjacent theopening 312. Dielectric members 320 and 322, such as a polypropylenedielectric tape, prevents contact of bars 306 and 308 by the movablepanel member 316. An alternately high and low impedance structure isdefined with the low impedances provided in the regions of theconductive bars 306 and 308 and the intermediate section 314 defining ahigh impedance which can be approximately 60 times the characteristicimpedances of the low impedance sections.

In this embodiment the conductive bars are modified in accordance withthe invention to provide a series of capacitive slots 324 to inhibit thepropagation of modes directed in a direction running parallel to thelongitudinal bars peripherally around the access opening. In this viewthe direction of such undesired modes is viewed as propagating directlyinto the paper. To provide for the suppression of such energy, slots 324communicate with chambers 326 provided by hollowing out conductive bar308. The first conductive member 308 disclosed immediately adjacent tothe access opening 312 may be the only modified structure or, ifnecessary for adequate energy control, the next conductive bar member306 may be similarly modified, as indicated by dotted line 328.

Referring next to FIG. 22 an alternative door seal arrangementincorporating longitudinal bars extending adjacent to the periphery ofan opening is shown. Door assembly 330 is slidably disposed by suitablemeans such as track members 360 attached to the oven front walls. A modesupporting structure having five sections is formed by three parallelspaced conductive bar members 332, 334 and 336 secured to surface 338 ofa front wall 340 and extending peripherally around the access opening342 by means of a nuts and bolts arrangement 344. Dielectric materialmembers 346 and 348 are sandwiched between the conductive bar membersand are secured by appropriate means. Each of the dielectric members isdimensioned to extend beyond the conductive bar members when the accessdoor panel 350 is closed to prevent metal-to-metal contact.

Slots 352 are disposed, first, in the conductive bar member 332peripherally around the access opening. After determination of theleakage value of escaping energy from the oven enclosure a second set ofslots 354 may be required in bar member 334 to attenuate such energy. Athird set of slots 356 in bar member 336 will assure the maximumprotection against the leakage of electromagnetic energy around the dooropening when the apparatus is operative. The foregoing slots are againarranged to inhibit the propagation of energy modes extendinglongitudinally and peripherally around the door opening in the directionindicated by the arrow 358. Alternatively, the foregoing embodiments canbe fabricated with the modes supporting structures mounted directly onthe movable door panels to contact the oven enclosure walls surroundingthe access opening when the door assembly is in the closed position. Inaddition the slots may be provided in any desired manner on any desiredwall. In the foregoing embodiments involving elongated bars it is alsowithin the teachings of the invention to provide corrugated sectionsdefining alternating spaces and conductive member ribs extending in theperipheral direction in accordance with the structure illustrated inFIG. 16.

The present invention affords a novel means for inhibiting initiation ofmodes propagating transversely or obliquely to a desired direction ofenergy propagation. The provision of a frequency stop band by circuitparameter variations inhibits propagation of modes in one direction atan operating frequency and provides for propagation of energy at thisfrequency in a different direction. In addition to high frequencyheating apparatus, the invention is useful in systems includingoversized waveguides supporting many modes, industrial conveyorized ovenapparatus having elongated entrance and exit structures, transmissionand communication systems. It is equally applicable to energy seals withand without choke arrangements. The invention may also be utilized inembodiments operating at more than one desired frequency.

Numerous variations or modifications of the disclosed invention will beevident to those skilled in the art. It is intended, therefore, that theforegoing description of the invention and the illustrative embodimentsbe considered in the broadest aspects and not in a limiting sense.

We claim: .[.
 1. Heating apparatus comprising:an enclosure; means forenergizing said enclosure with energy having at least a predeterminedfrequency; said enclosure comprising at least a wall member having anaccess opening and a closure member for said access opening; and anenergy seal comprising at least portions of said wall and closuremembers to define an elongated energy mode supporting structure; saidmode supporting structure comprising means for substantially inhibitingthe propagation of energy at said frequency in one direction andsubstantially providing for propagation of said energy in a differentdirection..]. .[.2. The apparatus according to claim 1 wherein saidpropagating inhibiting and provision means comprise structuralvariations to provide points of maximum and adjacent points of minimumvariation in the energy propagating characteristics along at least aportion of said mode supporting structure..]. .[.3. The apparatusaccording to claim 2 wherein the distance between points of maximumvariation and adjacent points of minimum variation are less than oneeffective electrical wavelength of said frequency of energy within saidenclosure..]. .[.4. The apparatus according to claim 2 wherein at leastsome of said structural variations are periodic..].
 5. .[.The apparatusaccording to claim 2 wherein at least some of said structural variationsare aperiodic.]. .Iadd.Heating apparatus comprising:an enclosure; meansfor energizing said enclosure with energy having at least apredetermined frequency and a plurality of modes; said enclosurecomprising at least a wall member having an access opening and a doorfor said access opening; an energy seal comprising at least portions ofsaid wall and door to define an elongated energy mode supportingstructure; said mode supporting structure comprising means for directingsubstantially all energy propagation entering said seal in a firstdirection outwardly across said mode supporting structure and means forsubstantially inhibiting the propagation of said energy at saidfrequency in directions perpendicular and oblique to said firstdirection; and said propagation inhibiting and directing meanscomprising aperiodic structural variations along at least a portion ofsaid mode supporting structure.Iaddend.. .[.6. The apparatus accordingto claim 2 wherein said structural variations are produced by meanscomprising slots..]. .[.7. The apparatus according to claim 2 whereinsaid structural have a major dimension extending normal to the directionof energy propagating along said structure..].
 8. .[.The heatingapparatus according to claim 6 wherein.]. .Iadd.Heating apparatuscomprising:an enclosure; means for energizing said enclosure with energyhaving at least a predetermined frequency; said enclosure comprising atleast a wall member having an access opening and door for said accessopening; an energy seal comprising at least portions of said wall anddoor to define an elongated energy mode supporting structure; said modesupporting structure comprising means for directing substantially allenergy propagation entering said seal in a first direction outwardlyacross said mode supporting structure and means for substantiallyinhibiting the propagation of energy at said frequency in directionsperpendicular and oblique to said first direction; said propagationinhibiting and directing means comprising structural variations along atleast a portion of said mode supporting structure; said structuralvariations being produced by means comprising slots; and .Iaddend. atleast some of said slots have a different length. .[.9. The apparatusaccording to claim 2 wherein one of said energy seal wall members ismovable..]. .[.10. The apparatus according to claim 1 and means coupledto said mode supporting structure for dissipating energy transmittedtherethrough..]. .[.11. Heating apparatus comprising: an enclosure;means for energizing said enclosure with energy having at least apredetermined frequency; means for producing a plurality of energy modeswithin said enclosure in a cyclically varying manner; said enclosurecomprising at least a wall member having an access opening and a closuremember for said access opening; and an energy seal comprising at leastportions of said wall and closure members to define an elongated energymode supporting structure; said mode supporting structure comprisingmeans for substantially inhibiting the propagation of energy at saidfrequency in one direction and substantially providing for propagationof said energy in a different direction..]. .[.12. The apparatusaccording to claim 11 wherein said propagation inhibiting and provisionmeans comprise structural variations to provide points of maximum andadjacent points of minimum variation in the energy propagationcharacteristics along at least a portion of said mode supportingstructure..]. .[.13. The apparatus according to claim 12 wherein thedistance between points of maximum variation and adjacent points ofminimum variation are less than one effective electrical wavelength ofsaid frequency within said enclosure..]. .[.14. The apparatus accordingto claim 11 wherein at least some of said structural variations areperiodic..]. .[.15. The apparatus according to claim 11 wherein at leastsome of said structural variations are aperiodic..]. .[. . The apparatusaccording to claim 11 wherein said structural variations are produced bymeans comprising slots..]. .[.17. The apparatus according to claim 11wherein said structural variations have a major dimension extendingnormal to the direction of energy propagating along said mode supportingstructure..]. .[.18. The apparatus according to claim 16 wherein atleast some of said slots have a different length..]. .[.19. Theapparatus according to claim 11 wherein one of said energy seal wallmembers is movable..]. .[.20. The apparatus according to claim 11 andmeans coupled to said mode supporting structure for dissipating energytransmitted therethrough..]. .[.21. In combination:a cavity; means forsupplying said cavity with electromagnetic wave energy in a frequencyrange where the dimensions of said cavity are substantially greater thana free space wavelength at said frequency; access means to said cavitycomprising a door for closing an opening in a wall of said cavity; andmeans for providing an energy seal between said door and said cavitywall comprising an elongated energy mode supporting structure; said modesupporting structure comprising means for substantially inhibiting theexcitation of predetermined modes of electromagnetic energy at saidfrequency peripherally along said structure and substantially providingfor the propagation of predetermined modes of said energy across saidstructure..]. .[.22. The combination in accordance with claim 2 whereinsaid excitation inhibiting and provision means comprise structuralvariations to provide alternately a maximum and a minimum point in theenergy propagating characteristics of said structure..]. .[.23. Thecombination in accordance with claim 22 wherein the distance betweenpoints of maximum variation and adjacent points of minimum variation areless than one effective electrical wavelength of said frequency..]..[.24. The combination in accordance with claim 23 wherein saidstructure comprises a dielectric medium..]. .[.25. The combination inaccordance with claim 24 wherein said structure comprises metallicbounding surfaces..]. .[.26. The combination in accordance with claim 25wherein said bounding surfaces defining said mode supporting structureare separated by said dielectric medium..]. .[.27. The combination inaccordance with claim 26 wherein a medium is provided adjacent saidopening for dissipating radiated energy escaping across saidstructure..]. .[.28. Heating apparatus comprising:an enclosure; meansfor energizing said enclosure with energy having a predeterminedfrequency; said enclosure having an access opening; a member for closingsaid opening; and elongated energy mode supporting structure comprisinga choke disposed adjacent to at least one peripheral wall of saidopening; said mode supporting structure comprising structural variationsto yield predetermined maximum and minimum points to control the energypropagating characteristics in predetermined directions within saidstructure..]. .[. . The apparatus according to claim 28 wherein saidchoke and mode supporting structure are filled with a dielectricmedium..]. .[.30. The apparatus according to claim 28 wherein said chokeis disposed along at least portions of the peripheral edges of saidclosure member..]. .[.31. The apparatus according to claim 28 whereinsaid choke is disposed within at least portions of one peripheral wallof said opening..]. .[.32. The apparatus according to claim 28 whereinat least some of said structural variations are periodic..]. .[.33. Theapparatus according to claim 28 wherein at least some of said structuralvariations are aperiodic..]. .[.34. The apparatus according to claim 28wherein the distance between points of maximum variation and adjacentpoints of minimum variation are less than one effective electricalwavelength of said frequency of energy within said enclosure..]. .[.35.The apparatus according to claim 28 and means for producing a pluralityof modes within said enclosure in a cyclically varying manner..]. .[.36.The apparatus according to claim 28 and means coupled to said modesupporting structure for dissipating energy escaping from said line..]..[.37. Heating apparatus comprising:a source of microwave energy; meansfor concentrating energy from said source in a restricted region ofspace comprising a wall member having an access opening; the maximumlinear dimension of said space being at least a plurality or free spacewavelengths of the predominant frequency band of said energy and theperiphery of said access opening being greater than a plurality of freespace wavelengths of said energy; means for substantially preventing theescape of said energy from said region through said opening comprisingmeans for producing substantially standing waves from any portions ofsaid energy which pass through said opening and a plurality of frequencyresponsive means spaced around at least a major portion of said openingfor providing impedance variations to transmission of said energy aroundthe periphery of said opening; the average spacing between the maximaand/or minima of said impedance variations at said frequency being lessthan one-half of a free space wavelength of said frequency..]. .[.38.The apparatus according to claim 37 wherein said means for preventingthe loss of said energy through said opening comprises a member forclosing said opening..]. .[.39. The apparatus according to claim 37 andsecondary means for preventing the escape of said energy comprising alossy energy absorbing material disposed adjacent to said opening..]..[.40. The apparatus according to claim 37 wherein said means forproviding impedance variations comprises spaced impedancediscontinuities..]. .[.41. the apparatus according to claim 40 whereinsaid impedance discontinuities comprise spaced slots extending in adirection substantially perpendicular to the periphery of saidopening..]. .[.42. The apparatus according to claim 40 wherein saidimpedance discontinuities comprise spaced slots extending obliquely tothe periphery of said opening..]. .[.43. The apparatus according toclaim 40 wherein said impedance discontinuities extend in a directionperpendicular to the periphery of said opening for a distancesubstantially equal to a quarter wavelength of said standing waves..]..[.44. The apparatus according to claim 37 wherein said means forproducing said standing waves comprises a substantially short circuitelectrically spaced from said wall member by a distance substantiallyequal to one or more half wavelengths of said standing waves..]. .[.45.The apparatus according to claim 38 wherein said means for providingsaid standing waves comprises a mode supporting structure made up atleast in part of portions of said wall member and portions of saidclosure member..]. .[.46. The apparatus according to claim 45 whereinsaid impedance variations extend around at least a major portion of atleast one peripheral wall of said mode supporting structure..]. .[.47.The apparatus according to claim 45 wherein said mode supportingstructure comprises an input section communicating with said restrictedregion and having said means for producing said impedance variations anda section outside said input section for reflecting an impedance whichis substantially less than the average characteristic impedance of saidinput section..]. .[.48. The apparatus according to claim 37 whereinsaid wall member is a portion of a conductive enclosure surrounding saidregion and said means for preventing the escape of said energy throughsaid opening comprises a conductive closure member covering said openingand spaced from said wall member by a dielectric medium forming a modesupporting structure having at least an input section and a chokesection and said means for providing impedance variations comprise aplurality of impedance discontinuities in the input section of said modesupporting structure spaced along a major portion of the periphery ofsaid opening with the average of said spacing being less than one-halfof a free space wavelength of said frequency..].
 49. .[.The apparatusaccording to claim 48.]. .Iadd.Heating apparatus comprising:a source ofmicrowave energy; means for concentrating energy from said source in arestricted region of space comprising a wall member having an accessopening; the maximum linear dimension of said space being at least aplurality of free space wavelengths of the predominant frequency band ofsaid energy and the periphery of said access opening being greater thana plurality of free space wavelengths of said energy; means forsubstantially preventing the escape of said energy from said regionthrough said opening comprising means for producing substantiallystanding waves from any portions of said energy which pass through saidopening and a plurality of frequency responsive means spaced around atleast a major portion of said opening for providing impedance variationsto transmission of said energy around the periphery of said opening; theaverage spacing between the maxima and/or minima of said impedancevariations at said frequency being less than one-half of a free spacewavelength of said frequency; said wall member being a portion of aconductive enclosure surrounding said region and said means forpreventing the escape of said energy through said opening comprises aconductive closure member covering said opening and spaced from saidwall member by a dielectric medium forming a mode supporting structurehaving at least an input section and a choke section and said means forproviding impedance variations comprising a plurality of impedancediscontinuities in the input section of said mode supporting structurespaced along a major portion of the periphery of said opening with theaverage of said spacing being less than one-half of a free spacewavelength of said frequency; and, .Iaddend. wherein said input andchoke sections are separated by a common wall and said spaced impedancediscontinuities are disposed in said common wall. .[.50. The apparatusaccording to claim 48 and secondary means for preventing the escape ofsaid energy comprising a lossy energy absorbing material coupled to theoutput portion of said mode supporting structure..]. .[.51. Heatingapparatus comprising: an enclosure; means for energizing said enclosurewith energy having a predetermined frequency; said enclosure comprisinga wall member having an access opening and a closure member for saidaccess opening; and an energy seal comprising at least portions of saidwall and closure members spaced from one another by a dielectric medium;said energy seal further comprising a plurality of spaced slotsextending in a direction substantially perpendicular to the periphery ofsaid opening and defining therebetween conductive elements; the averagespacing between said slots being less than one-half of a free spacewavelength of said frequency..]. .[.52. The apparatus according to claim51 wherein said spaced slots are disposed along at least portions of theperipheral edges of said closure member..]. .[.53. The apparatusaccording to claim 51 wherein said spaced slots are disposed along atleast portions of one peripheral wall of said opening..]. .[.54. Heatingapparatus comprising:an enclosure; means for energizing said enclosurewith energy having a predetermined frequency; said enclosure comprisinga wall member having an access opening and a closure member for saidopening; and an energy seal comprising an input section and a chokesection made up of at least portions of said wall and closure membersspaced from one another by a dielectric medium; said energy seal furthercomprising a plurality of spaced slots extending in a directionsubstantially perpendicular to the periphery of said opening anddefining therebetween conductive elements; said elements all having oneend interconnected and the other ends terminating in a dielectricmedium; the average spacing between said slots being less than one-halfa free space wavelength of said frequency..].
 55. .[.The apparatusaccording to claim 54.]. .Iadd.Heating apparatus comprising:anenclosure; means for energizing said enclosure with energy having apredetermined frequency; said enclosure comprising a wall member havingan access opening and a closure member for said opening; an energy sealcomprising an input section and a choke section made up of at leastportions of said wall and closure members spaced from one another by adielectric medium; said energy seal further comprising a plurality ofspaced slots extending in a direction substantially perpendicular to theperiphery of said opening and defining therebetween conductive elements;said elements all having one end interconnected and the other endsterminating in a dielectric medium; the average spacing between saidslots being less than one-half of a free space wavelength of saidfrequency; and .Iaddend. wherein said input and choke sections areseparated by a common wall and said spaced slots are disposed in saidcommon wall.
 56. The apparatus according to claim .[.54.]. .Iadd.55.Iaddend.wherein said choke section is disposed along at least portionsof one peripheral wall of said opening.
 57. The apparatus according toclaim .[.54.]. .Iadd.55 .Iaddend.wherein said choke section is disposedalong at least portions of one peripheral wall of said closure member..Iadd.58. Heating apparatus comprising:an enclosure; means forenergizing said enclosure with energy having a frequency in apredominant frequency band; the sum of the linear dimensions of saidenclosure being at least a plurality of free space wavelengths of saidfrequency; an access aperture in a wall of said enclosure; a door forclosing said access aperture; a microwave energy seal having conductiveportions of said wall and said door forming an elongated energy modesupporting structure comprising a reentrant choke structure having acommon wall with an input transmission line structure, said common wallbeing substantially less than a quarter wavelength in thickness at saidpredetermined frequency; and said common wall containing slots whichinhibit the propagation of said energy in directions in said sealperpendicular and oblique to an outward direction and direct thepropagation of said energy from the interior of said enclosure in saidoutward direction to said reentrant choke structure substantially onlythrough said input transmission line structure. .Iaddend. .Iadd.59.Heating apparatus comprising:an enclosure; means for energizing saidenclosure with energy having at least a predetermined frequency and aplurality of modes; said enclosure comprising at least a wall memberhaving an access opening and a door for said access opening; and anenergy seal comprising at least portions of said wall and door to definean elongated energy mode supporting structure; said mode supportingstructure comprising means for directing substantially all energypropagation entering said seal in a first direction outwardly acrosssaid mode supporting structure and means for substantially inhibitingthe propagation of energy at said frequency in directions perpendicularand oblique to said first direction; said propagation inhibiting anddirecting means comprising slots in said mode supporting structure, saidslots being arrayed in said perpendicular direction and extending insaid first direction. .Iaddend. .Iadd.60. Heating apparatus comprising:an enclosure; means for energizing said enclosure with energy having atleast a predetermined frequency and a plurality of modes; said enclosurecomprising at least a wall member having an access opening and a doorfor said access opening; an energy seal comprising at least portions ofsaid wall and door to define an elongated energy mode supportingstructure; said mode supporting structure comprising means for directingsubstantially all energy propagation entering said seal in a firstdirection outwardly across said mode supporting structure and means forsubstantially inhibiting the propagation of said energy at saidfrequency in directions perpendicular and oblique to said firstdirection; and said structural variations are produced by meanscomprising spaced slots extending obliquely to said perpendiculardirection. .Iaddend. .Iadd.61. A microwave oven comprising: a conductiveenclosure having an access aperture; a door for closing said accessaperture; means for supplying microwave energy predominantly in apredetermined band to said enclosure; means for cyclically varying themode pattern within said enclosure; the linear dimensions of saidenclosure being substantially greater than the free space wavelength ofsaid energy; a microwave seal comprising portions of a wall of saidconductive enclosure and said door; said seal comprising an inputtransmission line section disposed between the interior of saidenclosure and a choke section with means comprising slots in aconductive wall of said input transmission line section for directingsubstantially all of said energy propagation in a single directionoutwardly toward said choke section which reflects said energy backthrough said input section into said enclosure and for inhibiting thepropagation of energy in said frequency band in directions perpendicularand oblique to said single direction; and means for providing that allenergy entering said seal enters through said input transmission linesection. .Iaddend. .Iadd.62. A microwave oven comprising: a conductiveenclosure supplied with microwave energy predominantly in apredetermined band; said enclosure having an access aperture whoselinear dimensions are substantially greater than the free spacewavelength of said energy; a door for closing said access aperture;means for cyclically varying the mode pattern within said enclosure; amicrowave seal comprising conductive portions of said enclosure and saiddoor; said seal comprising an input transmission line sectioninterconnecting the interior of said enclosure and a choke section; andmeans comprising slots in a conductive wall of said choke section forrestricting propagation in said choke section to a TEM-like mode..Iaddend. .Iadd.63. A microwave oven comprising:a conductive enclosurehaving an access aperture; a door for closing said access aperture;means for supplying said enclosure with microwave energy predominantlyin a predetermined frequency band; means for cyclically varying the modepattern within said enclosure; the linear dimensions of said enclosurebeing substantially greater than the free space wavelength of saidenergy; a microwave seal comprising portions of a wall of saidconductive enclosure and said door; and said seal comprising an inputtransmission line section coupling the interior of said enclosure with achoke section with means comprising slots in a conductive wall of saidinput transmission line section for directing substantially all of saidenergy propagation in a single direction outwardly toward said chokesection which reflects said energy back through said input section intosaid enclosure and for inhibiting the propagation of energy in saidfrequency band in directions perpendicular and oblique to saiddirection, the input transmission line section comprising portions ofsaid wall and said door which form parallel elongated planar structuresand which limit energy propagation to said choke section to the spacebetween said planar structures. .Iaddend. .Iadd.64. A microwave ovencomprising: a conductive enclosure having an access aperture; a door forclosing said access aperture; means for supplying microwave energyperdominantly in a predetermined band to said enclosure; means forcyclically varying the mode pattern within said enclosure; the sum ofthe linear dimensions of said enclosure and said aperture beingsubstantially greater than the free space wavelength of said energy; amicrowave seal comprising portions of a wall of said conductiveenclosure and said door extending around the periphery of said accessaperture; said seal comprising an input transmission line sectioncoupled between the interior of said enclosure and a choke section;means for providing that all energy entering said seal enters throughsaid input transmission line section; and means comprising a slottedcommon wall of said input transmission line section and said chokesection for coupling to said choke section substantially all of saidenergy in said input transmission line section directed generallyorthogonally to the periphery of said access aperture while inhibitingthe propagation of said energy in the peripheral and oblique directionsof said access aperture. .Iaddend. .Iadd.65. A microwave ovencomprising:a conductive enclosure having an access aperture; a door forclosing said access aperture; means for supplying said enclosure withmicrowave energy predominantly in a predetermined band to saidenclosure; means for cyclically varying the mode pattern within saidenclosure; the linear dimensions of said enclosure being substantiallygreater than the free space wavelength of said energy; a microwave sealincluding portions of a wall of said conductive enclosure and said door;said seal comprising an input transmission line section disposed betweenthe interior of said enclosure and a choke section with means comprisingslots in a conductive wall of said input transmission line section forsubstantially restricting energy propagation in the input transmissionline section to a TEM-like mode directed toward said choke sectionwhereby said energy propagation is blocked by said choke section; andmeans for providing that all energy entering said seal enters throughsaid input transmission line section. .Iaddend. .Iadd.66. Heatingapparatus comprising: an enclosure; means for energizing said enclosurewith energy having at least a predetermined frequency and a plurality ofmodes; said enclosure comprising at least a wall member having an accessopening and a door for said access opening; and an energy sealcomprising at least portions of said wall and door to define anelongated energy mode supporting structure; said mode supportingstructure comprising means for directing substantially all energypropagation entering said energy seal in a first direction outwardlyacross said mode supporting structure and for substantially inhibitingthe propagation of said energy at said frequency in directionsperpendicular and oblique to said first direction. .Iaddend. .Iadd.67.The apparatus according to claim 66 wherein said propagation directingand inhibiting means comprises structural variations to provide pointsof maximum and adjacent points of minimum variation in the energypropagating characteristics along at least a portion of said modesupporting structure. .Iaddend. .Iadd.68. The apparatus according toclaim 67 wherein the distance between points of maximum variation andadjacent points of minimum variation are less than one effectiveelectrical wavelength of said frequency of energy within said enclosure..Iaddend. .Iadd.69. The apparatus according to claim 67 wherein at leastsome of said structural variations are periodic. .Iaddend. .Iadd.70. Theapparatus according to claim 67 wherein said structural variations areproduced by means comprising slots. .Iaddend. .Iadd.71. The apparatusaccording to claim 67 wherein said structural variations have a majordimension extending normal to the direction of energy propagating alongsaid structure. .Iaddend. .Iadd.72. The apparatus according to claim 67wherein one of said energy seal wall members is movable. .Iaddend..Iadd.73. The apparatus according to claim 66 and means coupled to saidmode supporting structure for dissipating energy transmittedtherethrough. .Iaddend. .Iadd.74. The apparatus according to claim 66wherein said plurality of energy modes are produced in a cyclicallyvarying manner. .Iaddend. .Iadd.75. Heating apparatus comprising:anenclosure; means for energizing said enclosure with energy having apredetermined frequency and a plurality of modes; said enclosure havingan access opening; a door for closing said opening; and elongated energymode supporting structure comprising an elongated choke disposedadjacent to at least one peripheral wall of said opening; said modesupporting structure comprising means for directing substantially allenergy propagation outwardly in a first direction to the mouth of saidchoke and for substantially inhibiting the propagation of energy at saidfrequency in directions perpendicular and oblique to said firstdirection, said propagation inhibiting and directing means comprisingstructural variations arranged in a direction perpendicular to saidfirst direction. .Iaddend. .Iadd.76. The apparatus according to claim 75wherein said choke and mode supporting structure are filled with adielectric medium. .Iaddend. .Iadd.77. The apparatus according to claim75 wherein said choke is disposed along at least portions of theperipheral edges of said closure member. .Iaddend. .Iadd.78. Theapparatus according to claim 75 wherein said choke is disposed within atleast portions of one peripheral wall of said opening. .Iaddend..Iadd.79. The apparatus according to claim 75 wherein at least some ofsaid structural variations are periodic. .Iaddend. .Iadd.80. Theapparatus according to claim 75 and means coupled to said modesupporting structure for dissipating energy escaping across saidstructure. .Iaddend. .Iadd.81. A microwave oven comprising:a conductiveenclosure having an access aperture; a door for closing and opening saidaccess aperture; means for supplying microwave energy predominantly in apredetermined band to said enclosure; means for cyclically varying themode pattern within said enclosure; the sum of the linear dimensions ofsaid enclosure being substantially greater than the free spacewavelength of said energy; a microwave seal comprising portions of awall of said conductive enclosure and said door; said seal comprising aninput transmission line section coupled between the interior of saidenclosure and a choke section; and means in a wall common to said inputtransmission line section and said choke section for directing thepropagation of substantially all of said energy in said frequency bandentering said seal in a single direction across said choke section andfor inhibiting the propagation of energy entering said seal indirections perpendicular and oblique to said single direction. .Iaddend..Iadd.82. An oven comprising:an enclosure; means for energizing saidenclosure with energy having a frequency in a predominant frequencyband; the sum of the maximum linear dimensions of said enclosure beingat least a plurality of free space wavelengths of said frequency; anaccess aperture in a wall of said enclosure; a door for closing saidaccess aperture and forming a microwave energy seal with said wall atsaid frequency band around the periphery of said aperture; said sealcomprising an input transmission line section coupled between theinterior of said enclosure and a reentrant choke section which providesfor the propagation of substantially all of said energy in saidfrequency band in a direction generally orthogonal to said peripheryfrom the interior of said enclosure substantially only through saidtransmission line structure to said reentrant choke structure whileinhibiting the propagation of waves of said energy in said frequencyband in directions perpendicular and oblique to said orthogonaldirection. .Iaddend. .Iadd.83. A microwave oven comprising: a conductiveenclosure having an access aperture; a door for closing said accessaperture; means for supplying microwave energy predominantly in apredetermined band to said enclosure; means for cyclically varying themode pattern within said enclosure; the sum of the linear dimensions ofsaid enclosure being substantially greater than the free spacewavelength of said energy; a microwave seal comprising portions of thewall of said conductive enclosure around said aperture and conductiveportions of said door; and said seal comprising an input transmissionline section, a choke section and an output transmission line sectionconnected at a common junction; said common junction being coupled tothe interior of said enclosure substantially only through said inputtransmission line section; and said seal comprising means in a thinconductive wall common to said input and choke sections for permittingthe propagation of waves of said energy entering said seal in an outwarddirection toward said choke section and for inhibiting the propagationof waves of said energy in said frequency band in directionsperpendicular and oblique to said outward direction. .Iaddend. .Iadd.84.A microwave oven comprising: a conductive enclosure having an accessaperture; a door for closing said access aperture; means for supplyingmicrowave energy predominantly in a predetermined band to saidenclosure; means for cyclically varying the mode pattern within saidenclosure; the sum of the linear dimensions of said enclosure and saidaperture being substantially greater than the free space wavelength ofsaid energy; a microwave seal comprising portions of a wall of saidconductive enclosure and said door around the periphery of said accessaperture; said seal comprising an input transmission line sectioncoupled between the interior of said enclosure and a choke section;means for providing that all energy entering said seal enters throughsaid input transmission line section; and means comprising an aperturedcommon wall of said input transmission line section and said chokesection for coupling to said choke section substantially all of saidenergy in said input transmission line section directed generallyorthogonally to the periphery of said access aperture while inhibitingthe propagation of said energy in the peripheral and oblique directions..Iaddend. .Iadd.85. Microwave heating apparatus comprising:an enclosure;means for energizing said enclosure with energy having a frequency in apredominant frequency band; an access aperture in a wall of saidenclosure; the sum of the linear dimensions of said enclosure and saidaperture being at least a plurality of free space wavelengths at saidfrequency; a door for closing said access aperture; a microwave energyseal between said wall and door comprising adjacent conductive portionsof said wall and closure members; said seal comprising a choke sectionand an input transmission line section having a thin conductive wallwhich is common to both sections and which contains means for providingfor the coupling of energy entering said seal from the interior of saidenclosure in an outward direction toward said choke section through saidinput transmission line structure while inhibiting the propagation ofsaid energy in directions perpendicular and oblique to said outwarddirection, said adjacent conductive portions forming parallel elongatedplanar structures with means limiting access to said choke section tothe space between said planar structures. .Iaddend. .Iadd.86. Amicrowave oven comprising:a conductive enclosure supplied with microwaveenergy predominantly in a predetermined band; said enclosure having anaccess aperture whose linear dimensions are substantially greater thanthe free space wavelength of said energy; a door for closing said accessaperture; means for cyclically varying the mode pattern within saidenclosure; a microwave seal comprising conductive portions of saidenclosure and said door; said seal comprising an input transmission linesection interconnecting the interior of said enclosure and a chokesection; and means in a conductive wall of said choke section forrestricting propagation in said choke section to a TEM-like mode andthereby inhibiting the propagation of other modes having propagationcomponents around the periphery of said access aperture. .Iaddend..Iadd.87. A microwave oven comprising: a conductive enclosure having anaccess aperture; a door for closing said access aperture; means forsupplying microwave energy predominantly in a predetermined band to saidenclosure; means for cyclically varying the mode pattern within saidenclosure; the linear dimensions of said enclosure being substantiallygreater than the free space wavelength of said energy; a microwave sealcomprising portions of a wall of said conductive enclosure and saiddoor; said seal comprising an input transmission line section disposedbetween the interior of said enclosure and a choke section with means ina conductive wall of said input transmission line section forsubstantially restricting said energy propagation to a TEM-like modedirected toward said choke section whereby said energy propagation isblocked by said choke section; and means for providing that all energyentering said seal enters through said input transmission line section..Iaddend. .Iadd.88. Heating apparatus comprising: an enclosure; meansfor energizing said enclosure with energy having a frequency in apredominant frequency band; the sum of the linear dimensions of saidenclosure being at least a plurality of free space wavelengths of saidfrequency; an access aperture in a wall of said enclosure; a door forclosing said access aperture; a microwave energy seal having conductiveportions of said wall and said door forming an elongated energy modesupporting structure comprising a reentrant choke structure having acommon wall with an input transmission line structure, said common wallbeing substantially less than a quarter wavelength in thickness at saidpredetermined frequency; and said common wall containing slots orientedin an outward-pointing direction and spaced from each other in adirection perpendicular to the outward-pointing direction. .Iaddend..Iadd.89. Heating apparatus comprising: an enclosure; means forenergizing said enclosure with energy having at least a predeterminedfrequency and a plurality of modes; said enclosure comprising at least awall member having an access opening and a door for said access opening;an energy seal comprising at least portions of said wall and door todefine an elongated energy mode supporting structure; and said modesupporting structure comprising slots in at least one of said wall andsaid door, the slots being arrayed in a peripheral direction around theaccess opening and extending in the outward-pointing direction, theextent of the slots in the direction perpendicular to both theperipheral direction and the outward-pointing direction beingsubstantially less than a quarter wavelength at said predeterminedfrequency. .Iaddend. .Iadd.90. A microwave oven comprising:a conductiveenclosure supplied with microwave energy predominantly in apredetermined band; said enclosure having an access aperture whoselinear dimensions are substantially greater than the free spacewavelength of said energy; a door for closing said access aperture;means for cyclically varying the mode pattern within said enclosure; amicrowave seal comprising conductive portions of said enclosure and saiddoor; said seal comprising an input transmission line sectioninterconnecting the interior of said enclosure and a choke section atleast in part bounded by a conductive wall, said conductive wall beingsubstantially less than a quarter wavelength in thickness at saidpredetermined frequency; and a plurality of slots in said conductivewall of said choke section, the slots being arrayed in a peripheraldirection around the access opening and extending in theoutward-pointing direction. .Iaddend. .Iadd.91. A microwave ovencomprising: a conductive enclosure having an access aperture; a door forclosing said access aperture; means for supplying with microwave energypredominantly in a predetermined band to said enclosure; means forcyclically varying the mode pattern within said enclosure; the lineardimensions of said enclosure being substantially greater than the freespace wavelength of said energy; a microwave seal comprising portions ofa wall of said conductive enclosure and said door; and said sealcomprising an input transmission line section disposed between theinterior of said enclosure and a choke section, with a plurality ofslots in a conductive wall of said input transmission line section, saidconductive wall being substantially less than a quarter wavelength inthickness at said predetermined frequency, and the slots being arrayedin a peripheral direction around the access opening and extending in theoutward-pointing direction. .Iaddend. .Iadd.92. Heating apparatuscomprising: an enclosure; means for energizing said enclosure withenergy having a frequency in a predominant frequency band; the sum ofthe linear dimensions of said enclosure being at least a plurality offree space wavelengths of said frequency; an access aperture in a wallof said enclosure; a door for closing said access aperture; a microwaveenergy seal having conductive portions of said wall and said doorforming an elongated energy mode supporting structure comprising aninput transmission line structure and a reentrant choke structure, theinput transmission line structure and the choke structure sharing aconductive wall in common, said conductive wall being substantially lessthan a quarter wavelength in thickness at said predetermined frequency;and said common wall containing a plurality of slots arrayed in spacedapart relationship in the peripheral direction around the access openingand extending in the outward-pointing direction. .Iaddend.